In directed infrared countermeasures (DIRCM) systems there has always been a problem of making the line-of-sight (LOS) to the target and the laser beam that is emitted from the DIRCM head coincident or parallel. While there have been many efforts to reduce the amount of parallax between the two and therefore reduce the aiming errors of the outgoing laser beam, the difficulty in the past has always been with the multiple mirrors that are utilized both for positioning the received image at the center of a focal plane array and for directing the laser beam out the DIRCM head.
In typical alignment procedures the multiple mirrors in the DIRCM head involve individual and independent adjustments. While it is possible for one pointing direction to adjust out the parallax by adjusting all the mirrors, the adjustment does not correct for all pointing directions. Since these mirrors move independently, the positional errors of the mirrors add up and are not easily compensated for over the entire field of regard. The situation is even further complicated as to how to lock the adjustment mechanisms without moving the adjustment while at the same time retaining boresight in view of the harsh environment.
Thus, in a cut and try operation one can spend an inordinate amount of time trying to compensate for the positional errors of these mirrors for all pointing directions; but in general these efforts have not met with success.
It is required that the angular error between the line-of-sight to the target and the laser path be extremely accurate and not exceed the laser divergence. There are a number of accumulated tolerances that must be compensated for i.e. mirror and support inaccuracies, focal plane array mounting inaccuracies, and gimbal fabrication tolerances. An objective in some DIRCM designs is to ensure that parallax allocation between the line-of-sight to the target and the laser beam path is less than 200 microradians, which is an exceedingly aggressive requirement. It is even more daunting because of the independent movement of the mirrors involved, noting that the angular errors accumulate for the various pointing directions. Moreover, compensating these mirrors for a single direction does not compensate the system for the entire field of regard.
There is therefore a need for a simplified gimbaling system that assures that if one detects the target image that appropriate movement will occur to insure the laser will in fact illuminate the target with laser energy and not miss it.
It would be noted that for gimbals the first step for the gimbal and the mirrors associated with it in a semi-spherical field of regard is to reflect the target image through a telescope and onto a focal plane array which is sensitive to the desired spectrum of light. The telescope not only focuses the target image but provides the means to measure angular displacement. Once the image is received on the focal plane array the displacement from the center is calculated and interpreted into angular displacement. This angular displacement is used to drive the azimuth and elevation stages of the gimbal, and associated mirrors, to drive the target image to the center of the focal plane array. Once the image is in the center of the focal plane array the laser is fired, with the assumption that the laser beam is parallel to the line-of-sight to the incoming image. This is where the problem arises. If there is error in parallax beyond the divergence of the laser beam the energy will not hit the target and the missile may not be jammed.
In one particular DIRCM system the head housing the gimbal is stable and the camera is fixed to the head. The result is that all of the pointing is done with movable mirrors. Note that the mirrors in the past have been adjustable with respect to the housing to which the camera is fixedly attached.
Once the target image reaches the camera, this target image is imaged onto a focal plane array so that the image is a particular spot within the field of view. The mirrors associated with acquiring the target image are physically moved so that the image spot is moved to the center of the focal plane array as described above.
Once the mirrors have centered the image at the center of the focal plane array, the laser is fired such that the direction of the laser beam corresponding to the center point of the array goes out coincident with or parallel to the line-of-sight to the target. If there were no pointing errors, then the outgoing laser beam would be coincident with or parallel with the line-of-sight to the target, and the laser beam would hit the target.
However, and as described above, it is difficult or impossible to post align the incoming image with the exiting laser beam after the gimbal has been assembled due the accumulation of errors throughout the assembly especially since a high degree of accuracy is required. This is where a simple and accurate system must be implemented to insure DIRCM functionality.